Lead-based perovskite compounds, such as lead zirconate titanate, have been widely used as piezoelectric ceramic materials because they have high piezoelectricity. Recently, however, piezoelectric ceramic materials which contain no lead have increasingly been preferred from the viewpoint of environmental friendliness. For example, there has been extensive research and development of lead-free perovskite alkali metallate(V) compounds, as typified by alkali metal niobates.
Anisotropic perovskite compounds, which are known to show different piezoelectric characteristics in different crystal-axis directions, can provide superior piezoelectric characteristics if their crystal axes are oriented. Accordingly, alkali metal niobate-based piezoelectric materials are expected to provide superior piezoelectric characteristics if their crystal axes are oriented.
One known method for orienting the crystal axes of an anisotropic perovskite compound with a high degree of orientation is discussed in Patent Document 1.
Patent Document 1 proposes a method for producing a crystal-oriented ceramic which includes a step of mixing a first anisotropically shaped powder whose growth plane (plane of the largest area) has lattice matching with a particular crystal plane of a predetermined first perovskite alkali metallate(V) compound and a first reaction material that reacts with the first anisotropically shaped powder to produce at least the first perovskite alkali metallate(V) compound; a step of molding the mixture prepared in the mixing step so that the first anisotropically shaped powder is oriented; and a heat-treatment step of heating the compact prepared in the molding step so that the first reaction material reacts with the first anisotropically shaped powder.
According to Patent Document 1, the first anisotropically shaped powder is produced by the following method.
The first anisotropically shaped powder is produced by heating, in a flux, a second anisotropically shaped powder whose growth plane is a {001} plane and which is formed of a bismuth-based layered perovskite compound represented by the general formula (Bi2O2)2+(Bi0.5AMm−1.5NbmO3m+1)2− (where m is an integer of 2 or more and AM is at least one alkali metal element selected from the group consisting of sodium, potassium, and lithium) and a second reaction material that reacts with the second anisotropically shaped powder to produce at least a second perovskite alkali metallate(V) compound.
That is, the plate-like powder of a layered perovskite compound functions as a reactive template for producing an anisotropic perovskite compound. If a perovskite alkali metallate(V) compound, such as potassium niobate, is prepared using a bismuth-based layered perovskite compound as a reactive template, A-site components, such as bismuth, contained in the reactive template inevitably remains as impurities in the resultant perovskite alkali metallate(V) compound. This makes it difficult to achieve desired superior piezoelectric characteristics.
However, the redundant A-site components contained in the bismuth-based layered perovskite compound (second anisotropically shaped powder) can be removed as redundant components by optimizing the compositions of the bismuth-based layered perovskite compound and the second reaction material and heating them to their eutectic point, or melting point, in a flux.
According to Patent Document 1, the bismuth-based layered perovskite compound is first used as a reactive template to prepare the first anisotropically shaped powder with the A-site components removed therefrom. The first anisotropically shaped powder is then used as a reactive template to prepare an oriented perovskite alkali metallate(V) compound. The bismuth-based layered perovskite compound with the optimum composition is produced by a method involving heat treatment, such as the flux method or hydrothermal synthesis.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-12373 (as in Claims 2 and 3 and Paragraphs [0064] and [0070]-[0075]).